This invention relates to tunable cavity resonators utilising micro electromechanical systems (MEMS) structures in the form of deformable suspended membranes suitable for, but not limited to, photonic purposes involving infrared (IR) electromagnetic radiation, and a method for fabricating the same.
The invention has particular, although not exclusive, utility in the fabrication of semiconductor devices that can be used as tunable detectors, emitters and filters for wavelength division multiplexing (WDM), micro-spectrometry and other purposes.
Aspects of the invention may be employed as multi or hyperspectral systems, wavelength agile detector systems and detector systems that are blind to countermeasures.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge in art as at the priority date of the application.
MEMS represent the integration of mechanical elements, sensors, actuators, photonics and electronics on single substrates. The use of microfabrication technology employed in silicon very large scale integration (VLSI) integrated circuit technology to generate miniature three-dimensional structures such as motors, gears, accelerometers and pressure sensors on the same substrate as the associated drive and sensing electronics, has dramatically expanded the field of mechatronics. In addition to allowing mechanical manufacture on a micro-miniature scale, MEMS technology brings the low-cost, high throughput advantages of VLSI technology to mechanical and sensor systems.
The use of MEMS structures in the fabrication of tunable cavity resonators to form a deformable suspended membrane is known, albeit for filtering applications in a very restrictive subset of the optical spectrum of wavelengths. The fabrication of these resonator devices has generally involved constructing the MEMS structure on a silicon substrate using technologies such as bulk micromachining and surface machining. Further, the use of such resonator devices has generally been limited to applications in which the wavelength of resonance need only be tuned a small amount (in the order of 0.01 to 0.10 μm). This in turn has limited the application of such resonator technology to short wavelengths (less than 1.6 μm), i.e. in wavelength regions of near IR and below. In the context of IR filtering technology, these wavelengths are less than 2 μm, and thus these types of resonator devices are quite unsuitable for applications in the short wavelength infrared (SWIR), the mid-wavelength infrared (MWIR), the long wavelength infrared (LWIR) and visible wavelength regions.
The inventors of the present invention have discovered one of the reasons for this is that, generally, surface micromachining techniques rely on high temperature (>700° C.) deposition processes such as low-pressure chemical vapour deposition (LPCVD) to produce high quality membrane films such as silicon nitride. However, the resultant membranes tend to be formed with considerable tensile stress. This tensile stress results in structures that are mechanically very stiff, preventing the large displacement needed for IR Fabry-Perot (FP) resonator applications, particularly in the short-wavelength infrared (SWIR—1.6-2.5 μm), mid-wavelength infrared (MWIR—3-5 μm) and long-wavelength infrared (LWIR—8-12 μm) regions.
Hence, it has previously been understood in the art that the use of MEMS structures in the fabrication of tunable cavity resonators is only applicable to very small wavelength applications, limiting the scope of the technology to near IR radiation and below for tunable lasers and detectors for short wavelength (e.g. less than 1500 nm optical communications) applications, and is not really suitable for longer wavelength radiation applications in the IR wavelength regions.
Resonant semiconductor FP cavity devices that have been fabricated for shorter wavelength applications using high temperature deposition processes generally have a fixed reflector layer that acts as a reflector on the substrate and a supported flexible membrane also having a reflector layer that acts as another reflector, the flexible membrane and the substrate being separated by an air gap. Tunability is achieved by deflecting the membrane through the application of a force. This force can be applied by various means, such as by way of a voltage applied to piezoelectric microactuators affixed to the membrane, or by applying a voltage across a pair of electrodes, one being, or surmounted to, the fixed reflector and the other being, or surmounted to, the reflector on the flexible membrane.
One method of constructing these FP cavity microstructure devices requires the use of a sacrificial etch layer, which is used to temporarily support the membrane during the fabrication process. This layer is removed via an etching stage in the fabrication step to release the membrane. The membrane layer itself consists of a film, such as silicon nitride film, upon which reflector layers, such as Au/Cr or distributed Bragg reflectors, are later deposited to form a reflector.
SiO2 is commonly employed as the sacrificial layer for LPCVD silicon nitride microstructure devices and is generally etched using HF or buffered HF (BHF), as both of these etchants demonstrate high selectivity when etching SiO2 sacrificial layers from silicon nitride membranes formed using LPCVD.
A concomitant problem with using high temperature deposition processes is the controllability of the intrinsic stress in the membrane during the fabrication process, quite aside from the issue of the high tensile stress that may be provided in the membrane after fabrication is completed. Stress control in suspended membranes is important as the intrinsic stress in the membrane directly determines the sensitivity of the device to any applied force. Too much intrinsic tensile stress results in stiff membranes, which in extreme situations can cause membranes to fracture, whilst compressive stresses result in buckled and/or collapsed membranes.
Therefore, in any application of MEMS structure technology to longer wavelength filtering, stress control is a major problem that has to be overcome.
Control is also critical in the fabrication of multi-layered structures, such as distributed Bragg reflectors. In these multi-layered applications, any unaccounted for residual stress can easily result in stress mismatch between the layers, leading to bowing or buckling of the released membrane.
Furthermore, in optical applications where FP cavity devices require a reflecting surface, the uniformity and flatness of the layer are of critical importance. Thus stress control during the fabrication of the membrane is important to ensure the layer remains flat and uniform on release.
Most photonic systems require a material system that has a direct and easily modifiable bandgap. This allows efficient optical generation and detection, while control of the bandgap makes possible bandgap engineering for enhanced device performance and allows use of quantum mechanical effects for new devices, such as photoconductors, photodiodes, avalanche photodiodes, bolometers, and others.
In order to access a large range of optical wavelengths, a wide variety of Group III-V and Group II-VI semiconductor systems including AlGaN/GaN, AlGaAs/GaAs, InAlGaAsP/InP, InGaP/GaP, HgCdTe/CdTe and ZnTeSe/ZnTe have been developed. A common feature of these materials (with the exception of AlGaN/GaN) is the requirement to undertake processing at temperatures significantly lower than those offered by the LPCVD process. Due to the differences in material properties and process technology, different process approaches must be used for each of these materials, significantly adding to the problem of MEMS development for systems integrating photonic devices.
There are many applications in the optoelectronics area, including IR applications, requiring wavelength tunable detectors and/or emitters. Examples of these include: high bandwidth optical communication systems utilising WDM technology; infrared detectors such as photoconductors, photodiodes and avalanche photodiodes; and micro-spectrometry to name but a few. This wavelength tunability can be achieved through the use of an FP cavity device. However, a simple and versatile method for fabricating a tunable FP cavity device with a moveable membrane having requisite stress characteristics and quality that can be used in longer photonic wavelength applications with wavelengths extending from less than 1 micron to greater than 20 microns, and which can be fabricated at relatively low temperatures has tended to have eluded the art to date.
It is an object of the present invention to provide for the fabrication of a tunable cavity resonator having a moveable suspended membrane and which can be formed on a variety of semiconductor systems to access a range of optical wavelengths, including infrared.
It is a preferred object of the invention to provide for an improved method of fabricating a tunable resonant cavity with a moveable and suspended membrane at a low deposition temperature.
It is a further preferred object to provide for the fabrication of a tunable cavity resonator having a moveable suspended membrane that can operate with wavelengths in the infrared regions.
It is another preferred object of the invention to provide for controlling the stress in the membrane during the fabrication of a tunable cavity resonator to enable it to be formed on a variety of semiconductor systems.
In accordance with one aspect of the present invention, there is provided a method for fabricating a tunable cavity resonator having a pair of reflectors, one being disposed in fixed relationship to a substrate material and the other being a suspended moveable membrane disposed a cavity length from the one reflector, and a pair of electrodes either being constituted by the reflectors or being juxtaposed therewith, one electrode with the one reflector and the other electrode with the other reflector, the method comprising:
depositing a first reflector layer on a substrate to form the one reflector of the cavity resonator;
forming a sacrificial layer of a prescribed material having a high etch selectivity for releasing the membrane in a suspended and spaced relationship from the one reflector;
forming the membrane on the sacrificial layer using a deposition technique characterised by providing the required intrinsic stress in the membrane;
depositing a second reflector layer on the membrane to form the other reflector;
patterning the second reflector layer in accordance with a prescribed membrane geometry;
etching the second reflector layer and the membrane to achieve said prescribed membrane geometry; and
etching the sacrificial layer to release the membrane and suspend it in substantially parallel relation to the first reflector layer.
Preferably, the method includes the step of forming recesses for the support locations in the sacrificial layer down to the substrate, after forming the sacrificial layer and prior to forming the membrane thereon;
extending the membrane layer to cover the sacrificial layer and the exposed substrate within the recesses during the forming of the membrane;
extending the second reflector layer to cover the membrane layer within the recesses during the forming of the second reflector layer; and
etching the sacrificial layer to remove it from around the membrane, so that the membrane and reflector layers are disposed with the recesses are revealed to constitute the support structure for suspending the membrane and the second reflector above the substrate.
Preferably, the method includes initially etching the sacrificial layer to remove regions thereof down to said first reflector layer on the substrate exposed by said etching.
Preferably, the method includes protecting those regions of the sacrificial layer intended to function as the residual support structure of the membrane.
Preferably, the method includes finally etching the remaining unprotected regions of the sacrificial layer to release the membrane and suspend it by the support structure in substantially parallel relation to the first reflector layer.
Preferably, the substrate material is a semiconductor system that provides access to the optical wavelengths necessary for resonance purposes in the resonant cavity.
Preferably the cavity length corresponds to optical wavelengths in the infrared region.
Preferably, the displacement of the suspended moveable membrane is commensurate to the full cavity length, but is controlled to be marginally less than the full cavity length so that the cavity can accommodate such displacement without the membrane contacting the one reflector.
Preferably, the membrane is formed of silicon nitride.
Preferably, the sacrificial layer is formed of zinc sulphide.
Preferably, the substrate is formed from an infrared sensitive material.
Preferably, the substrate is formed of mercury cadmium telluride (MCT).
Preferably, the deposition technique for forming the membrane is PECVD.
Optionally, the electrodes are formed separately of the reflective layers.
Alternatively, the reflective layers are formed to function as electrodes.
Preferably, the second reflector layer is etched using an anisotropic etching process.
Preferably, the anisotropic etching process for the second reflector layer involves dry etching.
Preferably, the dry etching involves plasma etching.
Preferably, the plasma etching is reactive ion etching.
Preferably, the sacrificial layer is initially etched using an isotropic etching process.
Preferably, the isotropic etching process for initially etching the sacrificial layer involves dry etching.
Preferably, the protection of the support structures is provided by photoresist.
Preferably, the remaining unprotected regions of the sacrificial layer are finally etched using an isotropic etching process.
Preferably, the isotropic etching process for finally etching the sacrificial layer involves wet etching.
In accordance with another aspect of the present invention, there is provided a tunable cavity resonator fabricated according to the method defined in the preceding aspect of the invention.
In accordance with a further aspect of the present invention, there is provided a tunable cavity resonator comprising:
a substrate material;
a moveable membrane disposed in substantially parallel relationship to said substrate and suspended relative thereto at the periphery of the membrane by a support structure;
a pair of reflectors, one being a first reflector layer disposed in fixed relationship upon the substrate material and the other being a second reflector layer disposed on the suspended deformable membrane to form a resonant cavity, the reflectors being disposed a cavity length from each other;the membrane and one reflector being shaped in accordance with a prescribed membrane geometry; anda pair of electrodes either being constituted by the reflectors or being juxtaposed therewith, one electrode with the one reflector and the other electrode with the other reflector;wherein the suspended moveable membrane is of substantially uniform thickness and has an intrinsic stress to permit electrostatic displacement of the membrane over relatively large distances using relatively low voltages applied to the electrodes.
Preferably, the substrate material is a semiconductor system that provides access to the optical wavelengths necessary for resonance purposes in the resonant cavity.
Preferably the cavity length corresponds to optical wavelengths in the infrared region.
Preferably, the displacement of the suspended moveable membrane can be up to the full cavity length, which is correspondingly larger to accommodate such displacement without the membrane contacting the one reflector.
Preferably, the membrane is formed of silicon nitride.
Preferably, the support structures are formed of zinc sulphide.
Preferably, the substrate is formed from an infrared sensitive material.
Preferably, the substrate is formed of mercury cadmium telluride (MCT).
In one embodiment, it is preferred that the electrodes are formed separately of the reflective layers.
In another embodiment, it is preferred that the reflective layers are formed to function as electrodes.
In accordance with another aspect of the invention, there is provided a method for fabricating a tunable cavity resonator for filtering incident radiation with longer photonic wavelengths from applications where the wavelengths may be in the order of less than 1 micron to applications where the wavelengths may be greater than 20 microns, the cavity resonator having a pair of reflectors, one being disposed in fixed relationship to a substrate material and the other being a disposed on a moveable membrane suspended by a support structure and disposed a cavity length from the one reflector, to form a Fabry Perot (FP) cavity, and a pair of electrodes either being constituted by the reflectors or being juxtaposed therewith, one electrode with the one reflector and the other electrode with the other reflector, to control the movement of the membrane in response to a prescribed dc voltage applied across electrodes, the method comprising:                forming the membrane on a sacrificial layer using a plasma deposition technique at a sufficiently low temperature with a gas flow ratio and prescribed RF frequency and power to excite the plasma so that the resultant stress in the membrane when released from the sacrificial layer and suspended by the support structure is controllable and very low, and the pinhole density in the membrane is sufficiently low to provide the membrane with the requisite integrity and morphology to exhibit a near-ideal Fabry Perot response.        
Preferably, the cavity resonator functions as a filter that is applicable for selecting incident radiation of wavelengths in ranges, whereby the wavelength of resonance can be in the region of 100 nm or 1,000,000 nm.
Preferably, the membrane is formed of silicon nitride.
Preferably, the sacrificial layer is formed of zinc sulphide.
Preferably, the substrate is formed from an infrared sensitive material.
Preferably, the substrate is formed of mercury cadmium telluride (MCT).
Preferably, the deposition technique for forming the membrane is PECVD.
Preferably, the temperature of the deposition is matched to the tolerance of the materials constituting the sacrificial layer and the substrate.
Preferably, the gas flow ratio is selected at the desired deposition temperature to achieve the requisite intrinsic stress at a prescribed RF frequency and power that still provides the membrane with the requisite integrity and morphology to maintain a high yield process.
Preferably, the gas flow ratio comprises an appropriate ratio of silane to ammonia to a diluting gas so as to achieve the required level of intrinsic stress that can result in a controlled low stress in the released membrane.
Optionally, the electrodes are formed separately of the reflective layers.
Alternatively, the reflective layers are formed to function as electrodes.
Preferably, the second reflector layer is etched using an anisotropic etching process.
Preferably, the anisotropic etching process for the second reflector layer involves dry etching.
Preferably, the dry etching involves plasma etching.
Preferably, the plasma etching is reactive ion etching.
Preferably, the sacrificial layer is initially etched using an isotropic etching process.
Preferably, the isotropic etching process for initially etching the sacrifical layer involves dry etching.
Preferably, the protection of the support structures is provided by photoresist.
Preferably, the remaining unprotected regions of the sacrificial layer are finally etched using an isotropic etching process.
Preferably, the isotropic etching process for finally etching the sacrificial layer involves wet etching.